1. Field of the Invention
The present invention relates to prealigners used to align a substrate prior to processing or testing and, in particular, to a high precision prealigner.
2. Discussion of the Related Art
In processing and measurement of semiconductor devices, the precise knowledge of the position, i.e., the center and orientation, of a substrate is often required. Prealigners are used to precisely position and orient articles in automated processing and metrology systems. For example, a prealigner can be used to align substrates, such as semiconductor wafers, for testing or processing during the manufacture of integrated circuits. Of course, prealigners may be useful in other industries in which the position of a substrate must be strictly controlled.
Current prealignment systems use a light source and corresponding detector to scan the periphery of a substrate while the substrate is rotated about an axis perpendicular to the plane of the substrate. Typically, the light source and detector used in prealignment systems are expensive and bulky components. Conventional prealignment systems determine the orientation of the substrate prior to loading the substrate on the processing or metrology stage. Once the center and orientation of the substrate is properly aligned, the system loads the substrate onto the processing or metrology stage.
In accordance with an embodiment of the present invention, a substrate is loaded on a rotary chuck and a prealigner locates the center of a substrate by performing an initial scan of the periphery of the substrate to generate an initial data set and analyzing the initial data set to determine the coarse position of the notch and the center offset of the substrate relative to the axis of rotation of the rotary chuck that the substrate is loaded on. The initial data set is curve fitted and the amplitude and phase of the curve are used to calculate the center offset. The prealigner also locates the apex of the notch by scanning a portion of the periphery of the substrate that includes the notch to generate a data set of the notch. The second data set is curve fitted and a derivative of the curve is generated. Using the derivative of the curve, e.g., the peaks on the derivative curve, the mid-point of the notch can be located and the offset error of the apex of the notch from the mid-point of the notch. The location of the apex of the notch can then be accurately determined using the calculated mid-point and offset error.
In another embodiment of the present invention, an apparatus includes a rotary chuck that holds a substrate by the periphery of the substrate and a prealigner system. The prealigner system includes a light source for producing light that will be at least partially incident on the periphery of the substrate and a light detector positioned to detect the light. The light detector converts the detected light to an output signal, which is received by a processor coupled to the light detector. The processor records the output signal of the light detector relative to the angular position of the substrate.
In one embodiment the prealigner further includes a computer program that is executed by the processor. The computer program includes instructions for fitting a curve to the data set of the recorded output signals of the light detector relative to the angular position of the substrate and for determining the derivative of the curve. The computer program also includes instructions for calculating the location of the apex of the notch using the derivative of the curve. In another embodiment, the computer program includes instructions for fitting a curve to the data set of the recorded output signals of said light detector relative to the angular position of said substrate determining the center offset of said substrate relative to said axis of rotation of said rotary chuck using the curve fitted to the data set.